But here's the punch line: Laughter and joy are not unique to humans, the study says. Ancestral forms of play and laughter existed in other animals long before humans began cracking up.

"Human laughter has robust roots in our animalian past," said Jaak Panksepp, a professor of psychobiology at Bowling Green State University in Ohio.

Panksepp has studied rats and found that when they "play," they often chirpa primitive form of laughter, according to the scientist. In an article to be published tomorrow in the journal Science, he makes the argument that animal laughter is the basis for human joy.

New research, however, shows that "circuits" for laughter exist in very ancient regions of the human brain.

As humans have incorporated language into play, we may have developed new connections to joyous parts of our brains that evolved before the cerebral cortex, the outer layer associated with thought and memory.

Researchers say that the capacity to laugh emerges early in child development, as anyone who has tickled a baby knows.

There is ample evidence that many other mammals make play sounds, including tickle-induced panting, which resembles human laughter. Indeed, animals are capable of many emotional feelings, just like humans, some scientists say.

"The recognition by neuroscientists that the brain mechanisms underlying pain, pleasure, fear, and lust are the same in humans and other mammals underscores our similarity to other species and is extremely important," said Tecumseh Fitch, a psychology lecturer at the University of St. Andrews in Scotland.